Fix excessively severe CSP URI error during first-time setup

Summary:
See D19394. Currently, during first-time setup before you configure "phabricator.base-uri", we may attempt to generate a setup page, try to generate a CSP header for it, and fail to access the environmental config. This causes a too-severe error page ("configure phabricator.base-uri") instead of preflight guidance (like "can't connect to MySQL").

Instead, treat this more like "security.alternate-file-domain" and just bail on CSP if we can't fetch it.

Test Plan: On a fresh (non-explodey laptop) install with critical setup errors (no MySQL installed yet), loaded Phabricator. Before: error about phabricator.base-uri. After: more helpful guidance about installing/configuring MySQL.

Reviewers: amckinley, avivey

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19396

src/aphront/response/AphrontResponse.php

@@ -113,6 +113,7 @@ abstract class AphrontResponse extends Phobject {
 
     try {
       $cdn = PhabricatorEnv::getEnvConfig('security.alternate-file-domain');
+      $base_uri = PhabricatorEnv::getURI('/');
     } catch (Exception $ex) {
       return null;
     }
@@ -124,8 +125,6 @@ abstract class AphrontResponse extends Phobject {
       // If an alternate file domain is not configured and the user is viewing
       // a Phame blog on a custom domain or some other custom site, we'll still
       // serve resources from the main site. Include the main site explicitly.
-
-      $base_uri = PhabricatorEnv::getURI('/');
       $base_uri = $this->newContentSecurityPolicySource($base_uri);
 
       $default = "'self' {$base_uri}";